625 
of Edinburgh, Session 1881 - 82 . 
In the apparatus already described I replaced the tubular carbon 
by a finely pointed piece, so as to have two fine carbon points 
exactly ojiposite each other. The resistance of the points was 
balanced in the bridge in the usual way. Pressure was then 
applied by a known number of turns, or parts of a turn, of the fine 
screw, and the change of resistance noted. The screw was then 
brought back to its former position and the pressure relieved, so as 
to allow the elasticity of the carbon to act, and restore the points to 
their first condition. It is obvious that if the change of resistance 
were due merely to elastic yielding, it should now be the same as 
before. This I found not to be the case. From the gritty nature 
of the carbon, the points of contact I found were perpetually 
changing, and hence the variation of resistance produced in this 
way obeyed no regular law. 
From this irregularity it is impossible, I think, to conclude that 
this cause could explain the transmission of musical sounds, far less 
articulate speech. 
As far as my experiments go, the following appears to be some- 
thing like the true explanation of the microphone action : — What I 
have termed the air and the tremor effects take place simultaneously. 
The tremor effect produces a jolting of the carbons, sufficient to 
allow momentary minute elastic arcs to take place between the 
points, which are just clear of contact with each other. Simul- 
taneously with this the air effect comes in, and on account of the 
variations of density due to the condensation and rarefaction of the 
air, acts uj>on the minute electric arcs so as to vary their resistance. 
The tremor effect explains merely the production of the musical 
pitch of the sounds heard in the telephone, whereas it is to the air 
effect that we must look for the transmission of the quality of the 
sounds uttered into the microphone transmitter. 
The microphone is thus so far a delicate make and break 
analogous to the old Reiss’ transmitter, with the important 
addition, however, of minute momentary gaps filled with a 
material which is sensitive to the harmonic variations of the 
atmospheric density which constitute sonorous vibrations. 
